Until recently, open celled foam structures have been manufactured almost exclusively from polyurethane thermosetting materials. D. Klempner and K. C. Frisch, Handbook of Polymeric Foams and Foam Technology, N.Y. (1991). Open celled thermoplastic foam structures have been produced by foaming as well as several other methods, such as voiding, leaching/extraction, thermally induced phase separation (TIPS) and phase inversion processes.
One way of making open celled foam structures without foaming is to prepare the specimen by uniformly blending a polymer resin and sized salt particles, and then leaching the soluble filler from the polymer matrix to create pores of different sizes, depending on the size of the salt particles. See C. R Thomas, Brit. Plast. 38, 552 (1965). Another leaching/extraction processes has also been disclosed in U.S. Pat. No. 3,745,202. U.S. Pat. No. 3,745,202 describes the use of extrusion of mixture of polymers and plasticizers to form a hollow fiber which is then stretched, and cured in air, in a water bath or in an aqueous solution of a plasticizer. This causes formation of porous hollow fiber. This is then subsequently treated with various solutions for treatment of the polymer and extraction of additives. This process is cumbersome, expensive and uses chemical solvents.
The thermally induced phase separation (TIPS) method involves forming a polymer solution with a leachable low molecular weight organic compound, followed by phase separation caused by rapid cooling and removal of the organic solvent by leaching. TIPS may be classified as liquid-liquid phase separation, liquid-solid phase separation and solid-liquid phase separation, where the first adjective describes the state of the polymer and the second adjective describes the low molecular weight organic compound. Shalaby et al., in U.S. Pat. Nos. 5,677,355 and 5,847,012 have modified the TIPS method to manufacture open cell foams. The method described by Shalaby involves co-melting a solid, crystalline fugitive organic compound like salicylic acid, naphthalene with an organic polymer to form an isotropic solution. This solution is then rapidly quenched using cryogenic techniques or water/air as a convection medium to form a foam precursor. This precursor which contains the dispersed fugitive compound was then treated by several techniques like leaching by solvent or sublimation through heating under vacuum. The resultant structure was determined to be a continuous open cell structure. This technique of manufacturing is cumbersome, involves a sequence of batch processes and does not yield a high cell density. Furthermore, it is not a continuous process and extraction of organic compounds needs to be done with care to prevent collapse. The TIPS process has also been described in U.S. Pat. Nos. 4,902,456 and 4,906,377 for manufacture of fluorocarbon porous films. Mrozinski in U.S. Pat. No. 5,238,623 describes use of TIPS along with an extraction process to achieve microporous polyolefin shaped article.
Kloos in U.S. Pat. Nos. 5,759,639 and 6,132,858 has described the manufacture of porous films using the phase inversion technique. Kloos describes the use of a polymer dope solution, which is coated on a substrate and then quenched in a nonsolvent, to produce a microporous structure. This technique is not attractive since it typically deals with organic solvents.
A voiding process to manufacture open cell structures has been disclosed in U.S. Pat. No. 4,877,679. Voiding, caused by the stretching of mineral filled polymers, has also been used to create an interconnected microporous structure by debonding at the mineral/polymer interface. See W. R. Hale, K. K. Dohrer, M. R. Tant, and I. D. Sand, Colloids and Surfaces A, 187, 483, (2001). Voiding can also be accomplished by stretching of immiscible polymers. U.S. Pat. No. 4,877,679 describes a unique process that combines voiding and extraction of plasticizer using a solvent to manufacture a microporous polyolefinic article. Requirements of this process are use of linear ultra high molecular weight polyethylene/polypropylene with dispersed insoluble filler and large amounts of plasticizer that is extracted during the manufacturing process. The stretching of mineral filled polymers has also been used to create an interconnected microporous structure by debonding at the mineral/polymer interface. See W. R. Hale, K. K. Dohrer, M. R. Tant, and I. D. Sand, Colloids and Surfaces A, 187, 483, (2001).
Another method of producing a foam structure is to use an interpolymer. Dow Chemical Co. developed Ethylene-Styrene Interpolymers (ESI) by copolymerization of ethylene and styrene monomers, and the open cell morphology of ESI can be easily controlled. See B. I. Chaudhary and R. P. Barry, Foams 99, 19-33 (1999).
Grafting is another common method to achieve an open celled foam structure. See K. Kaji, M. Hatada, I. Yoshizawa, and C. Kohara, J. Appl. Polym. Sci., 37, 2153-2164 (1989).
Recently, Kozma et al. introduced the silane grafting method (see M. L. Kozma, J. D. Bambara, R. F. Hurley, U.S. Pat. No. 5,859,076 (1999)). This technology is widely used in the foaming field. The blends of polyethylene and soft ethylene vinyl acetate (B. I. Chaudhary and B. A. Malone, U.S. Pat. No. 5,962,545 (1999)) or linear low density polyethylene (S. Abe, U.S. Pat. No. 6,414,047 B1 (2002)) were used to produce an open celled structure.
An open celled foam can also be created by opening up closed cells using a sharp needle. See S. D. Browers and D. E. Wiegand, U.S. Pat. No. 4,183,984 (1980).
These previous methods deal with foam structures having relatively large cell size. It has only been observed recently that microcellular open celled polycarbonate hollow fibers could be produced by adjusting the amount of injected CO2 and by changing the temperatures of the extruding head and nozzle. See Q. Huang, B. Seibig, and D. Paul, J. Membrane Sci., 161, 287-291 (1999); see also Q. Huang, B. Seibig, and D. Paul, J. Cellular Plast., 36, 112-125 (2000).
Polyurethane foams are typically produced by combining and reacting two hydroxyl-terminated compounds of polyol and polyisocyanate. See SPI Plastics Engineering Handbook of the Society of the Plastics Industry, Inc., 5th Ed. (1991). In general, high molecular weight, low functionality polyols produce a low amount of crosslinking, which leads to a flexible foam and vise versa. Therefore, by manipulating the formulation of two ingredients of polyurethane, any degree of flexibility can be achieved, and thereby, cell wall rupturing can be controlled.
Few thermoplastic foams with a very high open cell content are available because the manufacturing technologies for open celled thermoplastic foams have not been developed extensively. In the past, most thermoplastic foams have been produced in closed cells to preserve mechanical properties significantly while saving the material cost. However, such foams with isolated cells are not fit for applications requiring high permeability of gas or vapor, selective osmosis, and absorption and dampening of sound.
U.S. Pat. Nos. 6,051,174 and 5,866,053 disclose an extrusion system for providing a foamed material in which a material is supplied to an extruder for movement through a rotating screw member. The material is placed in a molten state and a foaming agent, such as a supercritical fluid, is introduced into the extruder at a selected pressure so that a two-phase mixture of the molten material and the foaming agent is formed. The foaming agent is then diffused into and dissolved in the molten material to form a single-phase solution which is forwarded from a solution formation to a nucleation device. A thermodynamic instability is induced through a rapid pressure drop, e.g., higher than 0.9 GPa/s in the nucleation device to nucleate microcells in the solution. A further shaping device, e.g., a die, can be used to produce a foamed material of a desired shape.
U.S. Pat. No. 5,334,356 discloses a supermicrocellular foamed material and a method for producing such material. The material to be foamed is a polymerplastic material, having a supercritical fluid, such as carbon dioxide in its supercritical state, introduced into the material to form a foamed fluid/material system having a plurality of cells distributed substantially throughout the material. Cell densities lying in a range from about 109 to about 1015 per cubic centimeter of the material can be achieved with the average cell sizes being at least less than 2.0 microns and preferably in a range from about 0.1 micron to about 1.0 micron.
If highly open celled foams with interconnections among adjacent cells can be produced in a cost effective manner, they will be used in numerous industrial applications such as, for example, filters, separation membranes, and diapers. In particular, fine celled or microcellular open celled foams will exhibit better properties for the applications requiring high permeability of gas or vapor, selective osmosis, and absorption and dampening of sound.